Coin operated machinery for vending goods or services in response to insertion of predetermined amounts of coin money are in widespread use both in the United States and throughout the world. Since one of the principal objects of constructing these machines is that they may be operated while unattended by the owner, the unfortunate, but inevitable, result has been that a large number of people attempt to cheat coin operated machines. Among the common forms of cheating, or attempting to cheat, coin operated machines are the use of slugs and the technique of "stringing".
The use of slugs is based on use of a non-coin piece of metal of a size identical to, or substantially similar to, the size of a valid coin. It is inserted into the machine in an attempt to operate it. Stringing is a cheating method whereby a piece of string is wrapped around the outer diameter of a coin and used to lower, and then attempt to remove, a coin from a vending machine so that the mechanism responds to insertion of the coin but the coin does not drop into the coin box.
Since the invention of the transistor, more and more vending machines are using electronic apparatus in coin validators. Among the advantages are greater reliability, and the fact that electronic coin validators may be designed to be much more immune to the use of slugs than many mechanical validators. For example, slide type mechanical coin chutes are virtually unable to detect slugs if the diameter and thickness of the slug is made the same as that of a valid coin. Prior art mechanical coin validators using falling coins had various arrangements for bouncing the deposited coin at the end of a fall down a runway because the densities of materials commonly used for slugs and valid coins tended to differ. Thus, the weight (as well as, in some cases, the elasticity) of a slug was different from that of a valid coin of the same dimensions. Most of these arrangements were limited to validators for accepting only one denomination of coin.
Electronic validators have provided various arrangements for detecting not only the diameter of coins but also electronic sensing means for detecting the metallic content of the coin as it traverses a predefined path along a runway through the validator.
Additionally, the use of modern electronics in coin validators has allowed arrangements where a single coin path for accepting all coins may be defined, but wherein the validator can detect the presence of a plurality of different denominations of coins having different metal content and different diameters.
As the construction of electronic validators for accepting coins of differing denominations has expanded, the arrangements for detecting various valid diameters have become more complex. For example, U.S. Pat. No. 4,249,648 to Myers discloses an electronic microprocessor driven arrangement wherein an optical lens and a light sensing array of elements are mounted next to a transparent portion of a chute carrying the coin, defining the predetermined path it travels through the validator.
As the coin enters the transparent portion, it becomes disposed between the light sensing array and a source of light. Periodically, at a high clock rate, contents of the shift register elements connected to the light sensing array (all of which is manufactured as a single unit) are shifted out and analyzed. When the trailing edge of a coin is detected by noting that the shift register is beginning to show a dark to light transition at the end corresponding to the physical front end of the transparent section, the contents of this scan of the shift register are analyzed to determined the diameter of the coin by the number of array elements which were darkened. Thus, a measure of the chord of the circle defined by the perimeter of the coin is made as it passes across the array.
U.S. Pat. No. 4,267,916 to Black et al. shows an arrangement using an array of light emitting diodes (LEDs) to measure chord length of a coin passing by the array. The apparatus detects coincidence between the covering of a particular LED of the array and a plurality of other LEDs in the array to determine a chord length. Circuitry requires a plurality of flip-flops and gates to detect the coincidence.
U.S. Pat. No. 3,653,481 to Boxall et al. shows an arrangement using four monostable multivibrators (one shots) per denomination of coin to detect coin diameters. The device disclosed in the Boxall patent uses a pair of one shots to perform each of two tests. The first test is for the length of time between the crossing of a first light emitting diode and the crossing of a second light emitting diode. The second test is directed to the time taken to initially cover and then uncover the second light-emitting diode. Since each one shot pair is set for a maximum and minimum acceptable value for a coin of a particular denomination, four one shots are required per denomination. A range of variations of the velocity of the coin as it travels down the path across the LED sensors must be "built in" to the timing periods of the one shots, so that variations in coin velocity do not adversely affect the device's accuracy. In essence, the Boxall apparatus requires a virtual constant velocity of coins of each denomination for it to operate properly. This requirement can lead to limitations on the angle at which the path may be disposed with respect to the local gravitational field for the device to work.
Since it is known in the art to use the powerful tool of microprocessors in electronic coin validators (see for example the Myer '648 patent referred to above), there is a need in the art to provide an improved diameter detecting apparatus which will reduce the number of components, external to the microprocessor, required to properly detect diameter, and which will be less sensitive to variations in coin velocity as it travels down the runway than apparatus such as that shown in Boxall.